The present invention concerns improvements in drilling equipment for use in horizontal drilling through rocks into strata of earth and sand.
Drilling of this kind below ground level is carried out for insertion of pipes for various purposes. Before blasting of tunnels, pipes thus are inserted into holes drilled one adjacent the other in the rock in an annular shape, to serve as casings for refrigerating pipes. The latter in turn serve the purpose of creating an artificial frost mass in the soil about the strata of earth that are later to be blasted away. The frost soil mass thus created in an artificial manner prevents water from flowing into the excavated tunnel, which saves times and effort.
Drilling equipment for use in such horizontal drilling operations comprises a drilling unit, said casing sections, a cutter head disposed at the forward end of the foremost casing as seen in the drilling direction, and drill rod sections which extend through the casing sections between the drilling aggregate and the cutter head and which are provided with an axial through channel feeding drill water to the cutter head.
The drilling operations involve several problems, however, particularly when the cutter head penetrates through rock into strata of earth and sand. If the air pressure inside the casing sections and the water pressure in the cutter head drill water channels are lower than the external ground water pressure, water and earth material will penetrate into said casing sections and channels all the way up to the drilling chamber. These displacements of material will in turn give rise to subsidence above the drilling space. In addition, material displacements also increase the risks of deviations from the intended drilling direction.
One has tried to eliminate this problem by using a cutter head, the diameter of which somewhat exceeds the internal diameter of the adjacent casing and which is mounted so as to close the forward casing end. Although this had a positive effect and partly prevented earth, sand, and water from penetrating, some other disadvantages arose in its stead. For instance, a cutter head of this size cannot be retrieved through the casing and used again. If, in addition, one fails to loosen the drill rod sections from the cutter head after completion of the drilling operation, which is not unusual, the drill hole cannot be used and a new hole must be started.
It is also possible to increase the air pressure inside the drill chamber for the purpose of preventing penetration of earth and water into the casing sections. However, this entails safety risks for the workmen and it is necessary to construct a sluice at the drill chamber entrance, which makes all transports to and from the drill chamber difficult and in addition time-consuming and expensive.
During the drilling operation the casing sections keep pace with the successive advancement of the cutter head through the strata of rock and earth. In rock drilling water does, however, seep into the drill hole also at the outside of these casing sections and flows into the drill chamber. When the cutter head reaches less compact strata of earth, the water is mixed with earth and sand, which aggravates the situation.
Attempts have been made to solve this problem by applying a seal in the form of an O-ring or a V-packing between the casing and the drill hole walls. These seals are, however, difficult to secure because of the movement of the casing in the working direction of the cutter head, and consequently the sealing effect is poor. On the other hand, if one succeeds in obtaining a comparatively good sealing effect, one instead has to face the disadvantage of accummulation of earth and sand forming plugs in front of the seal, which cause the casings to stick. In severe cases it might be necessary to drill a new hole for the casing adjacent the stuck one.